In many industrial processes, the concentration of trace species in flowing gas streams must be measured and analyzed with a high degree of speed and accuracy. Such measurement and analysis is required because the concentration of contaminants is often critical to the quality of the end product. Gases such as N.sub.2, O.sub.2, H.sub.2, Ar, and He are used to manufacture integrated circuits, for example, and the presence in those gases of impurities such as water--even at parts per billion (ppb) levels--is damaging and reduces the yield of operational circuits.
Consequently, many complex and sophisticated devices are available for measuring trace species in gases. They seek to detect low levels, on the order of a few parts per billion (ppb) by volume or less, with a short response time. As an example, electrochemical cells are used commercially to detect and monitor water contamination in high-purity process gas streams. These cells are limited in sensitivity to water levels around 30 ppb, have a relatively long response time, are poisoned by high transient concentrations of water, and are subject to interferences from other molecules.
I. Absorption Spectroscopy
Absorption spectroscopy offers higher sensitivity, response times on the order of microseconds, immunity from poisoning, and limited interference from other molecular species. Various molecular species, but especially simple molecules such as water, can be detected or identified by absorption spectroscopy. Thus, absorption spectroscopy provides a general method of detecting important trace species. In the gas phase, the sensitivity and selectivity of this method is optimized because the species have their absorption strength concentrated in a set of sharp rotational lines. Water is particularly suitable for absorption spectroscopy because the molecule has large rotational constants leading to a relatively small rotational partition function (q.sub.rotation =43 at room temperature). The narrow lines in the spectrum can be used to discriminate against most interfering species.
The relatively high sensitivity with which water can be spectroscopically monitored is important to manufacturers of high-purity gases used in the semiconductor industry. In most instances, parts per million (ppm) level detection is readily obtained. Detection sensitivities at the ppb level are attainable, in some cases, for water. Accordingly, several spectroscopic methods have been applied, including absorption measurements in traditional long pathlength cells, photoacoustic spectroscopy, frequency modulation spectroscopy, and intracavity loss spectroscopy. These methods have several features, to be discussed below, which make them difficult to use and impractical for industrial applications. They have been largely confined, therefore, to laboratory investigations.
The sensitivity of absorption-based methods is limited by the smallest effective sample absorption coefficient that can be measured. This in turn is limited by two factors: the noise equivalent fractional change in sample transmission and the effective pathlength through the sample. A single pass through the sample is limited by the physical dimensions of the apparatus, and is typically on the order of 0.1 to 1 meters.
A. Traditional Long-Path Spectroscopy
The traditional approach to increasing the absorbance pathlength (by on the order of 10-100 times) is to use a multipass cell to fold the light path many times through the cell. The most important design is that first reported by J. White, Long Optical Paths of Large Aperture, 32 J. Optical Soc'y Am. 285 (May 1942). Although many modifications of the original design have been published, White's original design is based upon three mirrors. A key feature of all the designs is the use of low F number optics (the F number is the ratio of the focal length to the limiting aperture of an optical system; the light throughput of such a system is proportional to 1/F.sup.2) and a placement of the mirrors at a separation that is on the edge between a stable and unstable optical cavity.
Both multipass cells and lasers can be treated as optical cavities which confine light with a certain loss per pass. Cavities are stable or unstable depending on the behavior of light rays slightly divergent from the optical axis of the cavity. These will remain close to the axis if the cavity in which they propagate is stable; in an unstable cavity, their distance from the axis will diverge exponentially. For a cavity on the edge between stable and unstable, the divergence is linear with the number of passes. Light is mode-matched into a cavity if the light beam has the right size and wavefront to exactly overlap itself on each round trip of the cavity.
The use of focusing optics at the edge between stable and unstable configurations results in a refocusing to a constant spot size on each round trip through the White cell and a running line of spots on one mirror. The pathlength is adjusted by changing the number of spots that fall on this mirror. Large mirrors are needed both to keep the light throughput high when such a cell is used with incoherent sources and to give ample space to resolve the spots used for coupling the light into and out of the cell. The maximum number of traversals of the cell is limited by both the reflectivity of the mirrors and by the need to physically separate the spots. Practical limits have been on the order of a few hundred passes of the cell, which translates to a maximum pathlength on the order of 0.5 km for cells of a few meters in physical length.
Special, very large cells have been constructed that allow pathlengths of several kilometers to be reached. These large cells are expensive, however, and difficult to control in temperature. Because they are not stable optical cavities, density gradients and turbulence caused by convective flow can easily destroy the optical beam quality and lead to substantial noise in the optical transmission. This is particularly a problem when the cells are used with Fourier Transform instruments, but also limits the sensitivity when used with coherent sources.
In recent years, novel cell designs based upon off-axis coupling into a marginally stable, near concentric optical cavity have been used. These designs are known as Herriott cells. D. Herriott et al., Off-Axis Paths in Spherical Mirror Interferometers, 3 Applied Optics 523 (April 1964). Light enters and leaves the cell through a single hole in one of the two mirrors. Unless the input light is carefully mode-matched, the spot size changes on each round trip, but it returns to its original size when it once again passes through the coupling hole.
The stable optical cavity makes the cell less sensitive to both mechanical vibration and convection than a White cell. One drawback is that the pathlength is varied by scanning the physical separation of the two mirrors and this can be somewhat difficult in a vacuum system. The effective F number of a Herriott cell is much greater than a White cell and, therefore, Herriott cells are not as efficient for incoherent sources. For laser sources, however, Herriott cells are superior. Like White cells, the maximum pathlength of the Herriott cell is limited by both the finite reflectivity of the large mirrors required (the beam attenuation through the cell is proportional to the mirrors' reflectivity raised to the power of the number of cell traversals) and by the requirement that, at each round trip, the beam forms a distinct, spatially resolved spot on one of the mirrors. For cells of practical size, these factors limit the total pathlengths to about 1 km or less.
B. Photoacoustic Spectroscopy
Photoacoustic or optoacoustic spectroscopy monitors absorption by its effect on the sample and, therefore, like laser induced fluorescence, is an indirect detection method. Following absorption of laser light, molecules collisionally relax, releasing heat which increases the local temperature. This local temperature rise leads to expansion, followed by contraction, as the heat energy is dissipated. Thus, by chopping a laser beam at an acoustical frequency (about 1 KHz), a synchronous sound wave will be produced which can be detected by a microphone. Photoacoustic cells are built to minimize sound waves produced by window absorption and may include a resonant inner section which allows the formation of acoustic standing waves at the laser chopping frequency. Microphones are extremely sensitive and, with a careful cell design, allow detection on the order of 10.sup.-8 W/cm of deposited energy. Because the deposited energy is measured, the signal (and thus the sensitivity) increases linearly with average laser power.
Consequently, photoacoustic spectroscopy is typically done with the sample cell inside the optical cavity of a 100% amplitude modulated, continuous wave laser to maximize the signal strength. A minimum noise equivalent absorption coefficient of 4.times.10.sup.-10 cm.sup.-1 has been obtained in this way. Efficient conversion of the heat to acoustic energy requires pressures of at least 10 to 100 torr, but this can be made up principally of buffer gas when the sample vapor pressure is too low.
The principal advantage of photoacoustic spectroscopy is its extreme sensitivity, the highest so far obtained for species that do not fluoresce. A number of drawbacks of photoacoustic spectroscopy are: (1) a quiet acoustic environment is required (therefore, use of an electric discharge or rapid flow of the sample leads to a substantial increase in noise); (2) the sample is exposed to high average light flux, which can lead to photochemistry in some situations; and (3) the indirect nature of the detection makes determination of absolute absorption strengths difficult. The only practical way to calibrate the strength of the acoustic signal is to use a mixture of a gas which has some transition whose cross-section is already known along with the gas of interest. Even with such calibration, uncertainties on the order of 20% remain.
C. Intracavity Loss Absorption Spectroscopy
A dye laser uses a solution of an organic dye that is optically pumped by another laser to produce gain. The principal advantage of these lasers is that they can be tuned over a broad spectral region. Dye lasers have a large gain bandwidth (the gain bandwidth is the spectral region over which a laser has net gain on each round trip).
When a laser with a large gain bandwidth is switched on, light intensity builds up from spontaneous emission. At first, the spectrum matches the gain bandwidth of the laser. The spectrum then begins to shift and narrow due to gain narrowing and then mode competition. If the laser contains a weak intracavity (i.e., inside the optical cavity) absorber, whose absorption features are narrow compared to the laser bandwidth, then as the time during which the laser has been pumped (t.sub.g) increases, narrow "holes" are eaten out of the broadband laser emission spectrum.
It has been established that, for the first 500 microseconds, the strength of these holes have an equivalent absorbance, A, of A=.alpha..times.f.times.c.times.t.sub.g, where .alpha. is the absorbance coefficient of the gas, f is the fraction of the laser cavity filled by the absorber (typically about 0.5), and c is the speed of light. Thus, the effective pathlength, L.sub.effective, is given by: L.sub.effective =f.times.c.times.t.sub.g, which can be up to a maximum of 100 km. Beyond this generation time, t.sub.g, mechanical vibrations and other sources of dephasing cause the depth of the absorption features to no longer follow a simple Lambert-Beer's law. See F. Stoeckel & G. Atkinson, Time Evolution of a Broadband Quasi-cw Dye Laser: Limitations of Sensitivity in Intracavity Laser Spectroscopy, 24 Applied Optics 3591 (Nov. 1985). Timing is achieved by the use of acoustooptic modulators on the pump and output of the laser, and the laser output is dispersed on a large spectrograph with an array detector. Using this technique, it is possible to measure spectroscopic features with an absorption coefficient as low as a few times 10.sup.-8 cm.sup.-1.
The sensitivity of this method is not quite as high as photoacoustic spectroscopy, but is much higher than that obtained with traditional long path cells. As long as one is careful to remain in the early time region, the absorption coefficient and, thus, the cross-section (assuming one knows the gas density and cavity fill factor), can be extracted from the optical depth of the observed absorption features. Because of the pulsed nature of the experiment, and the near threshold operation of the laser, chances for inadvertent photochemistry are reduced as compared to intracavity photoacoustic spectroscopy. The chief disadvantage is that this method requires complex instrumentation. A custom-designed, continuous wave dye laser must be used that has been carefully designed to remove all sources of stray interference which can lead to narrow bandwidth spikes on the laser output spectrum. In order to achieve Doppler limited resolution, the method requires a spectrograph of extremely high dispersion (about 10.sup.6) (a dimensionless number customarily defined as .DELTA..lambda./.lambda., where .DELTA..lambda. is the smallest change in wavelength that can be measured), which is an expensive piece of custom instrumentation.
D. Ring-Down Cavity Spectroscopy
The current "standard" way to use absorption spectroscopy to detect trace species in the gas phase to frequency modulate (FM) a laser source and detect a modulation of the amplitude of the laser radiation after passing through a sample gas. To increase sensitivity, a multiple pathlength cell is typically used. Although many designs exist, they are almost universally variants of two basic designs: the White and Herriott cells. As long as a weak laser source (where detector noise dominates) is not used, a ring-down cavity is superior in every respect over these designs.
In order to make sensitive absorption measurements, laser beam intensity fluctuations must be distinguished from molecular absorptions. This problem is particularly daunting for pulsed lasers whose shot-to-shot fluctuations are typically 5%. Although these fluctuations can be normalized out to some extent, absorptions on the order of 10.sup.-3 (dimensionless) are required to yield an observable signal.
A novel type of long pathlength cell, known as the ring down cavity (RDC) cell, was introduced as a sensitive gas phase direct absorption technique by A. O'Keefe & D. Deacon, Cavity Ring-Down Optical Spectrometer for Absorption Measurements Using Pulsed Laser Source, 59 Rev. Sci. Instrum. 2544 (Dec. 1988). This technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed cell cavity. By measuring the decay time (i.e., the exponential time constant which describes the time-dependent probability for loss of a photon from the cavity modes of a stable resonator), the technique avoids the problems mentioned above encountered with pulsed lasers.
Ring down cavity spectroscopy is based upon a simple idea which has become practical due to recent advances in reducing loss in dielectric mirrors. A ring down cavity is made from two highly reflective, concave mirrors aligned in a near confocal geometry as a stable, low-loss optical cavity (i.e., the mirror separation, d, is less than twice the radius of curvature). It is now possible to purchase at low cost ($100 each) small mirrors with reflectivity R&gt;99.99% over a range of some 60 nm anywhere in the visible spectrum, and mirrors with R=99.9998% have been reported near 840 nm (G. Rempe et al., Measurement of Ultralow Losses in an Optical Interferometer, 17 Optical Letters 363 (March 1992)).
Light from a conventional pulsed dye laser is coupled into the ring down cavity through one end. If the length of the laser pulse is less than the round trip time of the cavity (2d/c=10 to 20 nsec), then there can be no interference and a small but stable fraction of the incident light (about 10.sup.-5) enters the cavity. For a typical input pulse energy of a modest 1 mJ, this corresponds to about 3.times.10.sup.10 photons. These photons are trapped between the high reflectivity mirrors and slowly decay (ring down) due to the combined loss of the mirrors and any molecular absorber located between the mirrors. The empty cell has a decay time, .tau.=d/c(1-R). On each round trip, a fraction of approximately 10.sup.-5 of the intracavity light intensity is transmitted through the back mirror and is detected by a photomultiplier tube (PMT) or some other sensitive photodetector. With the time constant of the PMT set long compared to the round trip time, its output current follows a smooth exponential decay. This curve can be digitized and a least squares fit to extract the decay rate.
Effective pathlengths as large as 70 km with a cell two meters long, near the maximum attained by the intracavity loss absorption spectroscopy (ICLAS) method, can be achieved using ring down cavity spectroscopy. Pathlengths of near 1000 km can be attained with the best reported commercial mirrors. Shot-to-shot fluctuations in the laser intensity are normalized out. Noise levels of about 0.005 (2.sigma..sub.s), which corresponds to a noise equivalent absorption coefficient of about 8.times.10.sup.-10 cm.sup.-1, have been achieved. This is already quite competitive with the best that has ever been achieved by photoacoustic spectroscopy, and significantly better than the claimed sensitivity of ICLAS.
With mirrors of 1 ppm loss, a pathlength of 4000 km and noise equivalent absorption coefficient on the order of 10.sup.-12 cm.sup.-1 should be attainable, which is orders of magnitude better than has ever been realized in any absorption-based detection method. At one atmosphere pressure, this corresponds to an absorption cross-section of 3.times.10.sup.-32 cm.sup.2. Because the method is based upon light traveling a known distance through a passive optical cavity, Lambert-Beer's law should hold quantitatively for all pathlengths, and thus the method provides a direct determination of the optical extinction coefficient at a known pathlength.
Compared with the other methods discussed above, ring down cavity spectroscopy is the simplest and least expensive to implement. A ring down cavity spectroscopy system can be implemented for a cost on the order of $5,000/unit, at least a factor of ten less than the ICLAS method (the extreme expense of the ICLAS method has prevented many laboratories from adopting it). It is also the most flexible. It is important to point out that the optics used are of such high reflectivity and modest cost because they are of small size; the coated surfaces are less than 1 cm in diameter. The method samples only a narrow pencil of the sample, with a cross-section less than 1 mm.sup.2. Because a stable optical cavity is used, small deviations of the light beam due to density gradients will average out over several round trips, and should not contribute to noise as in traditional long pathlength cells. Furthermore, because the cross-sectional area of a ring down cell is much less than for a White cell, gas turbulence due to convection is much less likely to occur.
In summary, provided below is a list of some of the important advantages offered by the RDC cell over more traditional spectroscopic methods:
1) The RDC cell allows for much longer pathlengths. With both White and Herriott cells, practical considerations limit total pathlengths to about 100 times the physical length of the cell. With RDC cells, pathlengths of over 10.sup.4 times the physical length are easily achieved and, using the best reported commercially available mirrors, (1-R).apprxeq.2 ppm (where R=mirror reflectivity), pathlengths of 10.sup.6 times the cell length can be attained. PA1 2) The RDC cell is sensitive only to absorption loss between the mirrors. With FM spectroscopy, one is sensitive to absorption and reflection losses throughout the optical pathlength. For the detection of a ubiquitous environmental component, such as water vapor, this is a very important distinction. The absorption of water vapor in the housing of the diode laser limits detection sensitivity. High vacuum (&lt;10.sup.-5 torr) would have to be maintained throughout the entire optical path in order for external water vapor not to dominate over a sample with 10 ppb of water vapor. Another difficulty with FM spectroscopy is weak interference effects due to reflections in the cell and mirrors, which produce modulations of the optical transmission on the order of 1 to 0.1 cm.sup.-1, i.e., of the same characteristic width as absorption lines when broadened by atmospheric pressure. PA1 3) The pathlength (and thus the concentration accuracy) of a RDC cell is much higher. In the RDC cell, pathlength is determined by measurements of time, which are easily made at an accuracy of 1 ppm or better. In a White or Herriott cell, pathlength is determined by a complex path of the light beam through the cell. A slight misalignment of the cell will change the number of passes through the cell and, thus, the pathlength. As the number of passes in the cell is increased, it is ever more difficult to know the exact pathlength inside the cell. PA1 4) The RDC cell is much more compact. The light path inside the RDC cell is restricted to a Gaussian spot of less than 1 mm.sup.2 cross-section. Thus, a narrow tube, of less than 1 cm.sup.2 cross-section, can be used to contain the gas under study. In contrast, both White and Herriott cells use off-axis optical rays, and must have cross-sections on the order of 100 cm.sup.2 or greater to realize long effective pathlengths. The reduced volume of the RDC means that much less of potentially hazardous chemicals need to be held in the cell, which would need to be flushed out to make a new measurement, to check the instrument with a calibration sample, or at the end of the day. PA1 5) The RDC cell is much less sensitive to vibration and turbulence of gas inside the cell. A White cell is on the edge between a stable and unstable optical cavity. As a result, any deviation of a light ray, caused by either vibration of a mirror or index of refraction variation in the gas, will tend to accumulate on each pass, potentially producing a major source of noise. A Herriott cell is a marginally stable optical cavity and, therefore, has some averaging out of distortions, but as the pathlength is increased one moves closer to the edge of stability and the cell become more sensitive. The RDC is a near confocal cavity, and as such is the least sensitive of any optical design to misalignment and optical distortions. PA1 6) FM spectroscopy is ultimately limited by the amplitude noise in the laser source. Typical near IR diode lasers have amplitude noise that is 10.sup.2 to 10.sup.3 above shot noise. In the RDC, the decay of a passive cavity is measured when the pump source is turned off. Thus, laser noise does not contribute and a detection sensitivity limited essentially only by shot noise is obtained. In view of the advantages outlined above, the present: invention incorporates a ring down cavity cell.
II. Lasers
The term "laser" is an acronym for "light amplification by stimulated emission of radiation." As its name implies, a laser uses the principle of amplification of electromagnetic waves by stimulated emission of radiation. The essential parts of a laser are an amplifying medium, a source of pump power, and a resonator. The various types of lasers are classified according to their pumping or excitation mechanism: optically pumped lasers; gas-discharge lasers; pulsed gas lasers; chemical lasers; photodissociation lasers; nuclear lasers; gas-dynamic lasers; semiconductor lasers; free-electron lasers; and high-power, short-pulse lasers. A new type of laser is developed every year or so.
Lasers have rejuvenated spectroscopy because laser light far surpasses light from other sources in spectral or wavelength purity, intensity, coherence, and directionality (narrowness of beam). If required, laser light can be produced in extremely short and intense pulses. Lasers have increased the resolution and sensitivity of the conventional spectroscopic techniques discussed above.
Common commercial diode lasers, made from the III-V group of semiconductor materials, emit red and near-infrared wavelengths from about 0.63 to 1.55 .mu.m. Diode lasers have been used for spectroscopic studies and monitoring of important molecular species, including water. Most of this work has been confined, however, to the laboratory. Near-IR diode lasers have the advantages of single-mode outputs of milliwatts, near room temperature operation, and high energy efficiency, in addition to the availability of fiberoptic technology and inexpensive auxiliary equipment such as low-noise current drivers, thermoelectric coolers, collimating lenses, detectors, and optical isolators. D. Cooper & R. Martinelli, Near-Infrared Diode Lasers Monitor Molecular Species, Laser Focus World (Nov. 1992).
One significant drawback of these devices is that they are available in only relatively narrow spectral regions. Most vendors are either unable or unwilling to wavelength-select diode lasers for spectroscopic applications. Another drawback to near-IR diodes is that only a limited number of molecular species have absorption features in the spectral region covered by these lasers. Nevertheless, many molecules of interest--including water--have near-IR absorption bands that are strong enough for detection at ppm and, in some cases, ppb levels. Given the utility of diode lasers for trace species detection, commercial sources of lasers designed for such applications are becoming available. In addition, new technology, such as quantum well lasers, are dramatically increasing the spectral coverage of diode lasers.
III. RDCS and Laser Sources
As discussed above, O'Keefe and Deacon pioneered ring down cavity spectroscopy in 1988. A. O'Keefe & D. Deacon, supra. They discussed prior work using direct precision absorption measurements and noted that such measurements require sophisticated optical systems and sources which have a stable output intensity. The required intensity stability was achieved using several types of continuous lasers (e.g., infrared lasers, diode lasers, and tunable cw dye lasers) frequency noise. O'Keefe and Deacon stated, "The same degree of success has not yet been possible for experimental systems based upon pulsed laser systems for several reasons."
Accordingly, O'Keefe and Deacon developed a technique (RDCS) which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity (absorption losses of about 1 ppm per pass can be detected by O'Keefe and Deacon) significantly greater than that attained using stabilized continuous light sources. They describe the "key to the successful operation of this technique for optical absorption measurements" as "the use of a laser pulse with a coherence length so short that no interference can become established in the test cavity." O'Keefe and Deacon specifically disclose an amplified pulsed dye laser to drive their system.
Since 1988, O'Keefe and others have demonstrated that the RDCS technique can be used to perform sensitive absorption spectroscopy in a molecular jet expansion. A. O'Keefe et al., Cavity Ring Down Dye Laser Spectroscopy of Jet-Cooled Metal Clusters: Cu.sub.2 and Cu.sub.3, 172 Chemical Physics Letters 214 (Sept. 1990). The technique uses pulsed laser sources. Others have used RDC cells for a spectroscopic study of the stretching overtones in HCN and established that it is of comparable sensitivity to photoacoustic spectroscopy. D. Romanini & K. Lehmann, Ring-Down Cavity Absorption Spectroscopy of the Very Weak HCN Overtone Bands with Six, Seven, and Eight Stretching Quanta, 99 J. Chem. Phys. 6287 (Nov. 1993). The study used a pulsed dye laser. Still other investigators have recently shown that RDCS also can be used for quantitative kinetics measurements. T. Yu & M. Lin, Kinetics of Phenyl Radical Reactions Studied by the "Cavity-Ring Down" Method, 115 J. Am. Chem. Soc. 4371 (1993). Again, a pulsed dye laser was used. Finally, RDCS was extended to UV light and to incorporate a long coherence length laser and a short cavity in order to obtain high spectral resolution. G. Meijer et al., Coherent Cavity Ring Down Spectroscopy, 217 Chemical Physical Letters 112 (Jan. 1994). Although the authors extended the RDCS technique, they still used a pulsed dye laser.
To date, therefore, only short pulsed radiation sources (such as an excimer pumped dye laser) have been used to implement RDCS. This is because the pioneers of the RDCS technique, O'Keefe and Deacon, expressly required a pulsed radiation source. Pulsed lasers avoided the problem associated with the requirement of longitudinal mode coincidences by using short optical pulses so that every pulse of the laser enters the cavity with no additional intensity fluctuation or time delay. As a result, the sensitivity and data rate improve and the technical requirements of the system are relaxed.
Pulsed lasers are, however, large, bulky, and expensive. Although suitable for laboratory work, they are often impractical for field measurements. Accordingly, there remains an acute need for a relatively inexpensive and reliable device able to measure trace species at levels below 1 ppb by volume in industrial and environmental monitoring applications. The principle object of the present invention is to meet that need.